Don't reinstall a stale OpenClaw package over a working CLI

[Chessing234]

Summary

• Install > AI > OpenClaw only looked for the pacman package, so after pacman -R openclaw it re-offered a possibly older package that shadows a newer CLI on PATH and breaks a migrated state DB.
• Treat OpenClaw as present when the package, a PATH openclaw, or the gateway unit exists.
• Skip package install in that case, and warn when multiple openclaw binaries are on PATH.

Fixes #11829

Test plan

• bash test/shell.d/openclaw-present-test.sh
• With only an npm openclaw on PATH, Install > AI hides OpenClaw
• omarchy-install-openclaw-cli --now does not run pacman -S openclaw when PATH already has it
• Launching with two openclaw binaries prints a PATH warning

[smfworks]

Verified this against a machine living in exactly the scenario the PR targets (multi-binary OpenClaw install, package + PATH + gateway all present):

• omarchy-openclaw-present → exit 0, correct via all three branches: the openclaw 2026.9.4-1 pacman package is installed, command -v openclaw finds a CLI, and openclaw-gateway.service is enabled. /usr/bin/openclaw is the package's wrapper (exec node /usr/lib/node_modules/openclaw/openclaw.mjs "$@") coexisting with npm/mise installs — the stale-shadow scenario this PR exists for.
• omarchy-install-openclaw-cli --check → 0; --now → 0 with no pacman -S run. Correct skip behavior.
• The launcher's multi-binary detection fires on this machine with the real list: agent-cli wrapper, mise install, /usr/bin, /bin, mise shim — warning names the one bare openclaw resolves to. Exactly right.
• test/shell.d/default-agent-test.sh fully green (47 ok) — and removing the old openclaw mock is the correct call, since it would otherwise masquerade as a present install under the new presence logic.

One bug found: the new openclaw-present-test.sh is not host-hermetic. The fixture sets PATH="$tmp/bin:$ROOT/bin:/usr/bin:/bin", so on any machine with a real openclaw in /usr/bin — precisely the population this PR cares about — command -v openclaw finds the host binary and the first assertion fails:

not ok - absent OpenClaw should not count as present

The "absent" fixture (mocked omarchy-pkg-present → 1, mocked systemctl → 1, no unit file) is correct, but the host's binary leaks into the PATH check. Suggested fix: build the absent fixture from a controlled core-tools dir (symlinks to just the binaries the script and base-test.sh need) instead of the host /usr/bin:/bin, or otherwise mask any host openclaw for that scenario. Happy to verify a fix against this machine — it's a perfect reproduction host.

Interop note: the presence ladder also treats a mise/npm-installed CLI or gateway unit as present, so this composes cleanly with the npm-based install flow proposed in #7919 if that lands later.